1. In 1913, Niels Bohr proposed an atomic model of the hydrogen atom in which the electron orbits the nucleus in fixed, quantized energy levels.
2. Bohr's model explained the empirical formulas discovered by Balmer and Rydberg for the emission spectrum of hydrogen. It predicted that only certain orbital radii and electron energies were allowed.
3. Bohr's model resolved issues with the classical model, which predicted electrons would radiate energy and spiral into the nucleus. By quantizing angular momentum, Bohr was able to explain the stability of atoms.
6. Robert A. Millikan
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-
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Fg = mg
Fe = qE
1910-Millikan oil drop
e/me = - 1.758821011 C/kg
J.J. Thomson's experiment and the
charge-to-mass ratio of the electron
Newton's first law :
SF = 0 [Forces are Balanced]
9. Mass spectrometry (MS) is an analytical technique that ionizes chemical species and sorts the ions based
on their mass-to-charge ratio. In simpler terms, a mass spectrum measures the masses within a sample.
10. The Classical/Solar system
Atomic Model is doomed
• Let’s consider atoms as a quasi sun/planet model (only
one planet so that it is just a two body problem.
• The force balance of circular orbits for an electron
“going around” a stationary nucleolus
where v is the tangential velocity of the electron. Circular motion is
accelerated, accelerated charges need to radiate energy off
according to Maxwell, loosing kinetic energy
11. Figure : A prism spectroscope can be used to observe emission
spectra (a). The emission spectra of mercury (b) and barium (c)
show characteristic lines.
12. Figure : This apparatus is used to produce the absorption
spectrum of sodium (a). The emission spectrum of sodium
consists of several distinct lines (b), whereas the absorption
spectrum of sodium is nearly continuous (c).
13. Balmer Series
• In 1885, Johann Balmer found an empirical formula for
wavelengths of the emission spectrum for hydrogen in nm within
the visible range
(where k = 3,4,5…)
• A good physical theory needs to make sense of this empirical result
There is a minimum wavelength corresponding
to a maximum frequency and energy of photons
657nm
Red
486
Blue
434
Violet
410
397
365nm
Series
limit
14. Rydberg Equation
• As more scientists discovered emission lines at infrared and
ultraviolet wavelengths, the Balmer series equation was extended
to the Rydberg equation, actually on the basis of this equation,
people went out looking for more lines :
(where RH = 1.096776 x 107 m-1)
Hydrogen Series of Spectral Lines
Discoverer (year) Wavelength n k
Lyman (1916) Ultraviolet 1 >1
Balmer (1885) Visible, Ultraviolet 2 >2
Paschen (1908) Infrared 3 >3
Brackett (1922) Infrared 4 >4
Pfund (1924) Infrared 5 >5
Can be applied to isotopes of hydrogen by modifying R slightly
R
m
R
e
H
17
2
1009737.1
2
m
h
cm
R ea
Mm
Mm
e
e
+
Aside: